Zirconia (zirconium oxide) is used for a very wide range of applications. Typical examples of zirconia include powders produced by drying methods, powders produced by wetting methods, etc. Recently, research and development has been actively conducted on wet zirconia powders because of their multifunctionality. For example, wet zirconia produced by a wet purification method, such as hydrolysis, is used for electronic materials, co-catalysts for purifying automobile exhaust gas, oxygen sensors, fine ceramics, antireflection films, electrolytes of solid oxide fuel cells, and the like.
Wet zirconia is sometimes used as a powder base, but is often used as a sintered product to exhibit its functions. A zirconia sintered product is produced by molding a zirconia crystal fine powder, and then sintering the resulting molded product. In this case, the zirconia crystal fine powder is subjected beforehand to stabilizing treatment so as to maintain a tetragonal or cubic crystal structure, which is a high-temperature stable phase of zirconia crystal, to ordinary temperature. The stabilizing treatment of zirconia crystal is generally performed by dissolving oxides, such as calcia, magnesia, and yttria, in zirconia. Sintered products comprising zirconia having only a cubic crystal structure are widely used as so-called fully stabilized zirconia (generally referred to as “stabilized zirconia”) sintered products. Moreover, sintered products containing zirconia having a tetragonal crystal structure are widely used as partially stabilised zirconia sintered products.
When the above zirconia sintered product is obtained, the characteristics of the powder affect the handling and sinterability during production. Therefore, the characteristics of a zirconia sintered product to be obtained are greatly influenced by the characteristics of a zirconia powder used as a raw material. As a zirconia powder, for example, PTL 1 discloses a zirconia fine powder wherein the BET specific surface area is 6 to 28 m2/g, and the ratio of [average particle diameter measured by electron microscope]/[average particle diameter determined from BET specific surface area] is 0.9 to 2.1. This zirconia fine powder can be produced by controlling the average particle diameter of hydrated zirconia, which is a raw material, and the calcination temperature thereof. The moldability and sinterability when the powder is molded and sintered to form a ceramic are enhanced. Further, PTL 2 discloses a zirconia powder comprising secondary aggregate particles, wherein the BET specific surface area is 3.5 to 20 m2/g, the particle size median is 0.3 to 1 μm, and the ratio of [average particle diameter measured by electron microscope]/[average particle diameter determined from BET specific surface area] is 1 to 3. PTL 2 also discloses a zirconia powder and a method for producing the same, wherein the average particle diameter φ (μm) of hydrated zirconia sol is 0.2 μm or less, and this sol is calcined at a temperature T (° C.) that satisfies T≥3000φ+650 in the range of 800 to 1200° C. or 800 to 1300° C. to obtain a zirconia powder having a BET specific surface area S (m2/g) that satisfies the relationship φ≤1/S.
PTL 3 discloses a light-transmitting zirconia sintered product comprising zirconia containing 2 to 4 mol % of yttria as a stabilizer and less than 0.1 wt % of alumina as an additive, and having a relative density of 99.8% or more and a total light transmittance as measured at a thickness of 1.0 mm of 35% or more. PTL 3 further discloses a zirconia powder obtained by sintering a powder containing less than 0.1 wt % of alumina, and having a BET specific surface area of 10 to 15 m2/gm and an average particle diameter of 0.4 to 0.7 μm at atmospheric pressure in air, wherein the sintering shrinkage rate (Δρ/ΔT:g/cm3·° C.) during sintering at atmospheric pressure (in air, temperature rising rate: 300° C./h) is 0.0125 or more and 0.0160 or less. This powder is produced using hydrated zirconia sol as a starting material, and has a molding density of 50%.
PTL 4 discloses a zirconia-based porous body and a method for producing the same. Specifically, the zirconia-based porous body is produced by preparing a basic zirconium sulfate-containing reaction solution A by mixing a sulfidizing agent at 80° C. or more and less than 95° C. and a zirconium salt solution at 80° C. or more and less than 95° C., preparing a basic zirconium sulfate-containing reaction solution B by mixing a sulfidizing agent at 65° C. or more and less than 80° C. and a zirconium salt solution at 65° C. or more and less than 80° C., mixing the reaction solutions A and B, and aging the resulting mixture, followed by neutralization and firing.
PTL 5 discloses a porous zirconia-based powder and a method for producing the same. Specifically, when a sulfidizing agent is added to a zirconium salt solution, the sulfidizing agent is added to the zirconium salt solution at a temperature of 100° C. or more in an autoclave to thereby produce the porous zirconia-based powder.
The porous zirconia-based powders disclosed in PTL 4 and PTL 5 can be applied to catalyst carriers for purifying automobile exhaust gas, and can maintain a high pore volume even after high-temperature endurance.